Recently, high-density recording techniques have made a remarkable progress, which is greatly indebted to improvements in recording codes, signal detecting methods and signal processing methods including decoding methods as well as to the development of high-performance recording media and recording heads. Active studies of signal processing methods have been made for magnetooptical recording employing large-capacity rewritable memories.
The magnetic field modulating method for writing information on a magnetooptical disk is capable of overwriting and is suitable for high-density edge recording. It has been reported that high-density recording can be achieved by the combination of a sampled servo tracking method and a NRZI recording method (non-return-to-zero inverted recording method) as well as by the laser pulse magnetic field modulation method. Other known methods for high-density recording are a partial response (PR) method and a viterbi decoding method. Studies of a combination of a PR(1, 1) method, i.e., one of PR methods, and the viterbi algorithm using NRZI codes have been made.
The viterbi algorithm selects the most probable code string on the basis of information about the state of transition of reproduced signals (RF signals), which is a method that enables high-density recording through signal processing, differing from a physical method that depends on the reduction of the wavelength of the laser beam. Accordingly, the detection probability of the viterbi algorithm is higher than that of a method that detects bits individually.
In the optical disk, some recording pit patterns cause intersymbol interference and hence the amplitude of the RF signal takes a ternary value. Accordingly, the viterbi algorithm selects the most probable data string for detection on the basis of information about the state of transition of the ternary value.
However, when forming a ROM pit in an optical disk or forming a data pit by light modulation, the value of asymmetry of the RF signal varies according to the variation of the power of the writing laser beam; that is, the amplitude ratio between pit signals of ternary values varies. When the value of asymmetry varies, values detected by the viterbi algorithm include errors, which deteriorate the detection performance.